1. Field of the Invention
The invention relates to an input device, more particularly to an input device that is capable of ghost key suppression.
2. Description of the Related Art
FIG. 1 illustrates a conventional keyboard device. A switch module 1 of the keyboard device includes a plurality of strobe lines 111˜114, a plurality of sense lines 121˜124, and a plurality of switch units 13. Each of the switch units 13 includes a switch 131 electrically connected to a corresponding one of the strobe lines 111˜114 and a corresponding one of the sense lines 121˜124. When each of the switch 131 connected to the first strobe line 111 and the first sense line 121, the switch 131 connected to the first strobe line 111 and the third sense line 123, and the switch 131 connected to the third strobe line 113 and the first sense line 121 are at a conducting state, a ghost key ON state may result. In this case, in addition to the three switches identified above, the switch 131 electrically connected to the third strobe line 113 and the third sense line 123 may also be erroneously judged as having an ON state even though it is not actually at a conducting state. Further details may be found in U.S. Pat. No. 5,386,584.
In FIG. 2, a second conventional keyboard device capable of eliminating ghost keys is disclosed in U.S. Pat. No. 5,386,584. The second conventional keyboard device differs from the first in that each of the switch units 13′ further includes a diode 132 connected to the switch 131 in series. However, this configuration may increase the incurred cost of the keyboard device and prevent the switch module 1 from being printed on a flexible board.
U.S. Pat. No. 4,633,228 discloses a third conventional keyboard device capable of eliminating ghost keys. A switch module 2 of the keyboard device disclosed in the above patent includes a plurality of strobe lines 211˜214, a plurality of sense lines 221˜224, and a plurality of switch units 23. Each of the switch units 23 includes a switch 231 and a resistor 232 electrically connected in series to a corresponding one of the strobe lines 211˜214 and a corresponding one of the sense lines 221˜224. Each of the sense lines 221˜224 is electrically connected to a power supply source through a corresponding resistor 3. This keyboard device uses the voltage level generated by the resistors 232, 3 connected in series relative to a specific resistor 232 to determine whether a selected one of the switches 231 is at a conducting state. However, this keyboard device has the following disadvantages: (1) for a selected switch 231, when more switches are electrically connected, the equivalent circuit effect of the series-parallel connected resistors 232, 3 often yields incorrect results; (2) the conduction state of only one switch 231 can be determined at a time, which results in an extended time period being required to determine the conducting state of all the switches 231; (3) high precision resistances are required for the resistors 232, 3; and (4) each of the sense lines 221˜224 requires one comparator (not shown), and hence the number of comparators increases with the number of the sense lines 221˜224.